1. Technical Field of the Invention
The embodiments of the invention relate to wireless communications and, more particularly, to frequency offset estimation in wireless receivers.
2. Description of Related Art
Various wireless communication systems are known today to provide links between devices, whether directly or through a network. Such communication systems range from national and/or international cellular telephone systems, the Internet, point-to-point in-home systems, as well as other systems. Communication systems typically operate in accordance with one or more communication standards or protocol. For instance, wireless communication systems may operate using protocols, such as IEEE 802.11, Bluetooth™, advanced mobile phone services (AMPS), digital AMPS, global system for mobile communications (GSM), code division multiple access (CDMA), local multi-point distribution systems (LMDS), multi-channel-multi-point distribution systems (MMDS), as well as others.
For each wireless communication device to participate in wireless communications, it generally includes a built-in radio transceiver (i.e., receiver and transmitter) or is coupled to an associated radio transceiver (e.g., a station for in-home and/or in-building wireless communication networks, modem, etc.). Typically, the transceiver includes a baseband processing stage and a radio frequency (RF) stage. The baseband processing provides the conversion from data to baseband signals for transmitting and baseband signals to data for receiving, in accordance with a particular wireless communication protocol. The baseband processing stage is coupled to a RF stage (transmitter section and receiver section) that provides the conversion between the baseband signals and RF signals. The RF stage may be a direct conversion transceiver that converts directly between baseband and RF or may include one or more intermediate frequency stage(s).
Furthermore, wireless devices typically operate within certain radio frequency ranges or band established by one or more communication standards or protocols. The 2.4/5.8 GHz Bands that encompass current WiFi and Bluetooth™ protocols have limited data throughput. A newer 60 GHz standard, being developed by the Wireless Gigabit Alliance (WiGig or WGA) and IEEE TGad, pursues higher throughput of up to 7 Gbps in short-range wireless data transmissions. Using 60 GHz technology, high data rate transfers, such as real-time uncompressed/compressed high-definition (HD) video and audio streams, may be transferred between two or more devices. Some examples of transfers between two devices include data transfers between a conference room projector and a laptop, between a camcorder and a display, or between a network storage server and a laptop. Other examples abound. Due to this inherent real-time requirement for the targeting applications, 60 GHz standard explicitly defines a Quality of Service (QoS) requirement for traffic streams to meet high throughput among devices.
Frequency offset occurs between a transmitter and a receiver in a wireless communication. The frequency offset may be caused by a variety of factors, but two main causes are due to the mismatch between the transmitter local oscillator (LO) and the receiver LO, and the distortion of the transmitted signal in the channel during transmit. When the transmitted signal reaches the receiver, the receiver attempts to estimate this frequency offset, in order to provide compensation at the receiver for the offset between the transmitter and the receiver.
Traditional wireless local area network (WLAN) receivers use short and long preambles located at the beginning of each received packet to respectively estimate the coarse frequency offset Δfcoarse and fine frequency offset Δffine. The sum of the coarse and fine frequency offsets (denoted as Δfcoarse+fine) is then used as an estimation of the actual frequency offset (denoted as Δf) existing between the wireless transmitter and receiver. The receiver can conduct time domain phase rotation at the rate of Δfcoarse+fine to compensate for the frequency offset. Various techniques are known to provide for Δfcoarse+fine compensation. See for example, “An Improved Frequency Offset Estimator for OFDM Applications,” Michele Morelli and Umberto Mengali, IEEE Communications Letters, Vol. 3, No. 3, March 1999, pp. 75-77; and “Robust Frequency and Timing Synchronization for OFDM,” Timothy M. Schmidl and Donald C. Cox, IEEE Transactions on Communications, Vol. 45, No. 12, December 1997, pp. 1613-1621.
However, the actual offset is Δf=Δfcoarse+fine+Δfresidual, where Δfresidual denotes the residual frequency error after the Δfcoarse+fine compensation. The receiver may choose to track Δfresidual for the rest of the packet. For example, in OFDM systems, the receiver can extract the remaining phase error, denoted as εphase, from the pilot tones embedded in each data symbol for all the remaining data symbols in the packet and feed it into a phase-locked loop (PLL) based tracking loop. Integrated over all previous data symbols in the packet, the PLL outputs an estimate of Δfresidual (denoted as Δf*residual) for the current data symbol. See for example, “Residual Frequency Offset Correction for Coherently Modulated OFDM Systems in Wireless Communication,” V. S. Abhayawardhana and I. J. Wassell, University of Cambridge, UK, IEEE 55th Vehicular Technology Conference, 2002, pp. 777-781; and “Residual carrier Frequency Offset Tracking for OFDM-based Systems,” Chih-Peng Li, Po-Lin Chen and Tsui-Tsai Lin, The 2004 IEEE Asia-Pacific Conference on Circuits and Systems, Dec. 6-9, 2004, pp. 989-992.
Since current WLAN protocols use CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance) based wireless channel access mechanism, a WLAN receiver has no way of knowing whether the next packet is addressed to it or other receivers in the wireless network until the Receive Address field is decoded. Accordingly, the above mentioned frequency offset estimation and compensation scheme is limited within a single packet and the process is repeated all-over again at the beginning of the subsequent packet. A drawback of this approach is that Δfcoarse+fine is recalculated for each packet, even if the subsequent packet is from the same source as the previous packet. Another drawback is that the Δfresidual is left uncompensated at the beginning of a packet, since the Δfresidual tracking does not commence until later in the packet.
In stances where there is sufficient signal-to-noise ratio (SNR), the value of Δfresidual may not be significant to have a high impact in packet recovery. However, in low SNR situations, the Δfresidual may be significant due to the Δfcoarse+fine accuracy degradation. For systems operating in the 60 GHz frequency band, significant loss in link margin may occur as compared to similar systems operating at 2.4-5.8 GHz, or even lower frequency band, due to the significantly increased free space loss. As a result, 60 GHz systems typically rely on high antenna gains (via beamforming) to compensate for the loss in link margin in order to achieve useful range of operation.
In addition, many critical packet-related information fields are contained in the packet PLCP Header, which is located after the preambles. Large, uncompensated Δfresidual may reduce the chance of correctly decoding the PLCP Header, as well as the subsequent data. This is especially true for OFDM systems, due to the constellation rotation and inter-carrier-interference (ICI) between the sub-carriers introduced by uncompensated Δfresidual.
Accordingly, there is a need for having a robust frequency offset estimation and compensation scheme in low SNR situation for higher frequency bands, such as the 60 GHz band. This need is more so for applications requiring high QoS (Quality of Services), such as high definition audio/video under the new 60 GHz operational band standard proposed by WiGiG/IEEE. Because the retransmission of corrupted data packets not only wastes valuable wireless channel time but also introduces additional packet delivery latency, wherein such excessive retransmission and buffering due to repeated failures can ruin the user experience.